Nanoparticle-stabilized immiscible alloys

ABSTRACT

Solid immiscible alloys and methods for making the solid immiscible alloys are provided. The microstructure of the immiscible alloys is characterized by a minority phase comprising a plurality of particles of an inorganic material dispersed in a majority phase comprising a continuous matrix of another inorganic material. The methods utilize nanoparticles to control both the collisional growth and the diffusional growth of the minority phase particles in the matrix during the formation of the alloy microstructure.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/669,636 that was filed Nov. 6, 2012, the entire contents of whichare hereby incorporated herein by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under 70NANB10H003awarded by the National Institute of Standards and Technology. Thegovernment has certain rights in the invention.

BACKGROUND

Immiscible alloys with a miscibility gap in the liquid state arescientifically important and can offer unusual properties that may beuseful for a wide range of applications, such as bearings, electricalcontacts and switches, superconductors, and Giant Magnetoresistive (GMR)materials. However, it is challenging to effectively control thediffusional and colliding growth of immiscible minority droplets in animmiscible alloy during cooling in order to obtain a uniform dispersionof micrometer-sized or nanometer-sized minority phases in the alloyliquids and/or in the solids.

For an immiscible alloy, above the miscibility gap the alloy componentsare completely miscible as a single solution. If this single phaseliquid is cooled down into the miscibility gap, the single phase liquidbecomes unstable, nucleating and then separating into two liquid phaseswith distinct properties. The diffusion coefficient in alloy liquids isgenerally very high and, thus, after nucleation, the minority liquiddroplets can grow very rapidly to become large droplets, which are proneto coagulation (colliding growth) and segregation.

Conventional techniques for forming two-phase alloys utilize a rapidcooling rate to reduce the time for the diffusional and colliding growthof the minority liquid phases. Unfortunately, a high temperaturegradient during rapid cooling generally induces severe thermo-capillaryforces that push the droplets to the hotter region, making uniformdispersion and colliding growth control of the minority dropletsextremely difficult to achieve. Moreover, the use of high cooling ratesrestricts the size and complexity of the fabricated products, severelylimiting their penetration into technical applications.

SUMMARY

Immiscible alloy materials and methods for fabricating the immisciblealloy materials are provided.

In one embodiment, the immiscible alloy material comprises an immisciblealloy comprising a majority phase comprising a continuous matrix of afirst inorganic material; and a minority phase comprising a plurality ofparticles of a second inorganic material dispersed in the majorityphase, wherein the minority phase particles have an average diameter ofno greater than about 20 μm; and nanoparticles comprise a thermallystable material coating the surfaces of the dispersed minority phaseparticles, wherein the nanoparticles have an average diameter of nogreater than about 250 nm.

One embodiment of the methods of making the solid immiscible alloymaterials comprises: forming a single-phase liquid solution comprisingthe first inorganic material and the second inorganic material at atemperature above the liquid miscibility gap for the immiscible alloy;introducing solid nanoparticles comprising a thermally stable materialinto the single-phase liquid solution while it is at a temperature abovethe liquid miscibility gap; cooling the liquid solution through theliquid miscibility gap, whereby the nanoparticles form a coating onliquid droplets of the minority phase that forms in a liquid matrix ofthe majority phase; and cooling the resulting material to a temperaturebelow the solidus temperature, whereby the solid immiscible alloymaterial forms.

The first and second inorganic materials may comprise metals, metalloidsor other high-temperature materials. The nanoparticles are thermallystable in the first and second inorganic materials and may be, forexample, ceramics.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIG. 1A. Phase diagram for immiscible alloy Al—Bi with miscibility gap(L₁+L₂ region). FIG. 1B. Schematic of the structure evolution duringcooling in the absence of stabilizing nanoparticles. FIG. 1C. Growthcontrol of droplets by nanoparticles during cooling of the immisciblealloy.

FIGS. 2A-2F show the distribution of the Bi phase in solidified Al—Bialloys. SEM images of a pure Al20Bi alloy (FIG. 2A) and an Al20Bi alloywith 2 vol. % TiC_(0.7)N_(0.3) nanoparticles (FIGS. 2B and 2C) cooled atcooling rate of 1 K/s. Bi diameter (FIG. 2D) and Bi area fraction (FIG.2E) measured at different distances from the bottom of the sample. FIG.2F. Bi diameter and area fraction measured at different distances fromthe center of the Al20Bi alloy with 2 vol. % TiC_(0.7)N_(0.3)nanoparticles.

FIG. 3A. SEM image of a representative Bi droplet with TiC_(0.7)N_(0.3)nanoparticle coating. FIG. 3B Zoom-in of the squared area in FIG. 3A.FIG. 3C. Schematic of the interfacial energy as a function of thenanoparticle position relative to the liquid-liquid interface. FIG. 3D.Bi droplet size as a function of γ=S/C_(np) under 1K/s cooling. FIG. 3E.Bi droplet size as a function of cooling rate for Al-20Bi-2 vol. %TiC_(0.7)N_(0.3).

FIG. 4A. SEM image of a Bi droplet cluster. FIG. 4B. zoom-in of thesquared area in FIG. 4A. FIG. 4C. Schematic of the capillary pressure inthe liquid film between two colliding nanoparticle-coated droplets. FIG.4D. EDS of the interface region between two Bi droplets.

FIG. 5. TEM image of TiC_(0.7)N_(0.3) nanoparticle at Bi and Alinterface in an Al—Bi alloy. The wetting angle of 128° was measured fromthe triple junction.

FIG. 6. Schematic of a theoretical model for nanoparticle-induceddiffusional growth control.

FIG. 7 is a table showing the parameters used to calculate k_(np) and f.

FIG. 8 is a table showing the parameters used for the theoreticalcalculation of Bi size.

DETAILED DESCRIPTION

Solid immiscible alloys and methods for making the solid immisciblealloys are provided. The microstructure of the immiscible alloys ischaracterized by a minority phase comprising a plurality of particles ofan inorganic material dispersed in a majority phase comprising acontinuous matrix of another inorganic material. The distribution of theminority phase particles in immiscible alloys made using the presentmethods can be highly uniform, in contrast to the non-uniformdistribution of the minority phase particles in immiscible alloysfabricated using conventional methods, in which gravity induces asignificant amount of particle sedimentation and/or thermocapillaryforces induce segregation before the solidification of the alloy. Thepresent methods utilize nanoparticles to control both collisional growth(via coalescence) and diffusional growth of the minority phase particlesin the matrix during the formation of the alloy microstructure. As aresult, the size of the minority phase particles can be carefullycontrolled during alloy formation.

Immiscible alloys have a phase diagram characterized by the presence ofa liquid miscibility gap in which two liquid phases do not mix due theirlow solubilities in one another. In the miscibility gap, the majorityphase forms a continuous liquid phase and the minority phase formsliquid droplets dispersed in the majority phase. By way of illustration,FIG. 1A shows the phase diagram for Al—Bi. In this phase diagram, themiscibility gap is the region labeled (L₁+L₂). Above the miscibilitygap, the alloy components are miscible and form a single-phase liquidsolution. When this single-phase liquid solution is cooled down into themiscibility gap, it becomes unstable and separates into two separatephases.

Immiscible alloys can be composed of a variety of high-temperature andlow-temperature inorganic materials. As used herein, the phrasehigh-temperature inorganic material refers to an inorganic material thathas a melting point of at least about 100° C. More typically, thehigh-temperature inorganic materials have a melting point of at leastabout 300° C. This includes inorganic materials having melting points ofat least 500° C. and at least 1000° C.

High-temperature inorganic materials include metals, metal alloys(including alloys of metal and metalloid elements), intermetalliccompounds (i.e., compounds formed of two or more metals) and ceramics,including both oxide and non-oxide ceramics (e.g., carbides, nitrides,borides and silicides). Suitable alloys include binary and ternary andother, higher multicomponent alloys. The majority phase of theimmiscible alloys can be (or can comprise), for example, metals, such asaluminum, copper, gallium, zinc, iron or nickel. The minority phase canbe (or can comprise) such elements as bismuth, lead, silver, cadmium orsilicon. Thus, the immiscible alloy can be an aluminum-based alloy, suchas Al—Bi, Al—Pb, Al—In or Al—Si, in which aluminum forms the majorityphase. The amount of minority phase in the alloys can vary over a broadrange. For example, the minority phase may account for one, or even lessthan one, weight percent (wt. %) of the alloy or only a few weightpercent. However, in some alloys, the minority phase may account for upto 10 wt. %, up to 20 wt. %, up to 30 wt. %, or more of the alloy.

The stabilizing nanoparticles are composed of thermally stable inorganicmaterials. As used herein, the phrase thermally stable material refersto a material that remains a solid at a temperature above themiscibility gap of the immiscible alloy in which it is dispersed. Thethermally stable material can be, for example, a ceramic, a metal, ametal alloy or an intermetallic. Suitable ceramics include oxides,carbides, borides, nitrides and silicides. Specific examples includeTiCN, SiC, TiB₂, ZrO₂, and Al₂O₃. In order to provide a diffusioncontrol coating at the interface between the majority and minorityphases, it is desirable that the material selected for the nanoparticleshave sufficient wettability in the material of both alloy phases. Thus,the thermally stable material may be selected such it has a wettingangle of around 90° (such as from 20°-160°) on the liquid phase of eachmaterial in the presence of the other liquid material. This includesembodiments in which the thermally stable material is selected such thatit has a wetting angle of about 100° on the liquid phase of eachmaterial in the presence of the other liquid material.

As used herein, the term nanoparticle refers to a particle whosesmallest cross-sectional dimension is no greater than 1000 nm. A typicalnanoparticle in the present materials will have a smallestcross-sectional dimension of no greater than about 500 nm and moretypically, no greater than about 150 nm. Moreover, althoughnanoparticles having a smallest cross-sectional dimension at the largerend of the size range may be present, the distribution of nanoparticlesin the alloy material has a significantly lower average smallestdimension (as determined from the smallest cross-sectional dimension foreach nanoparticle in the distribution). Thus, in some embodiments, thedistribution of nanoparticles in the alloy material has an averagesmallest dimension of no greater than about 250 nm. This includesembodiments in which the distribution of nanoparticles has an averagesmallest dimension of no greater than about 200 nm, an average smallestdimension of no greater than about 150 nm, or an average smallestdimension of no greater than about 100 nm. For example, in someembodiments the average smallest dimension for the nanoparticles in thedistribution is in the range from about 10 to about 100 nm. In someembodiments, for example in embodiments in which the nanoparticles arespherical or substantially spherical, the largest dimensions of thenanoparticles (or the average largest dimension for the distribution ofnanoparticles) also fall within the ranges cited above.

The nanoparticles may have a wide range of morphologies. For example,the nanoparticles may be spherical or substantially spherical, elongated(e.g., nanorods or nanowires), disc-shaped, or sheet-shaped (e.g., inthe form of thin flakes).

The concentration of nanoparticles in the immiscible alloy should besufficient to provide a continuous or substantially continuous coatingover the surfaces of the minority phase particles. Although an excess ofnanoparticles can be used, any excess should be minimized to avoidunnecessary expense and/or a negative impact on alloy properties. Insome embodiments of the alloy materials, the nanoparticles account forno greater than about 10 vol. % of the alloy material, based on thetotal volume of the majority phase, the minority phase and thenanoparticles. This includes embodiments in which the nanoparticlesaccount for no greater than about 5 vol. % (e.g., from about 0.1 toabout 5 vol. %, including from about 1 to about 5 vol. %).

FIG. 1B illustrates the formation of an immiscible alloy in the absenceof stabilizing nanoparticles. As shown in panel (a), above themiscibility gap, the material exists as a single-phase liquid solution102 (represented by a star in the phase diagram of FIG. 1A). However, asthe temperature of the system is lowered along the solid vertical linein the phase diagram, nucleation begins (panel (b)) and the inorganicmaterials of the alloy separate into a continuous majority matrix phase104 in which droplets of a discontinuous minority phase 106 aredispersed. These droplets continue to grow in size (panel (c)) viadiffusional growth, where the droplet growth rate is controlled by therate at which the minority phase material diffuses through the majorityphase material. Eventually, the droplets begin to grow throughcollisional growth (panel (d)), whereby the droplets collide andcoalesce into larger droplets 108. Unimpeded, these growth processesoccur very rapidly and result in the formation of relatively largedroplets that tend to undergo sedimentation, such that the density ofthe minority phase of the immiscible alloys is significantly greater atthe bottom of the alloy sample.

FIG. 1C illustrates the use of nanoparticles as collisional anddiffusional growth control agents during the alloy solidificationprocess. Again, initially the material exists as a single-phase liquidsolution 202 (panel (a)). However, in the present methods, thestabilizing nanoparticles 203 are added to the solution while it isabove the miscibility gap. When the material is cooled into themiscibility gap, droplets of the minority phase 206 begin to separatefrom the inorganic material of the majority phase matrix 204 (panel(b)). Now, however, the nanoparticles spontaneously assemble at theinterfaces between the growing minority phase droplets and the majorityphase matrix material to form a coating 207 on the droplets, as depictedin panel (c). As a result, diffusional transport is restricted, or evencompletely blocked, and droplet 206 size is reduced relative to thedroplet size in the absence of the nanoparticle coating. Moreover, asshown in panel (d), nanoparticle coating 207 also serves as a barrieragainst droplet coalescence by introducing a capillary pressure in theliquid film between approaching nanoparticle-coated droplets, therebyfurther restricting the size of droplets 206.

Using the present methods, droplet size can be controlled such that itremains below the size at which gravitational and thermocapillary forcesbecome significant and create a non-uniform distribution. This is thecase even for slow cooling rates. In fact, the present methods can becarried out at cooling rates much lower than those used in rapid coolingtechniques, which are typically on the order of hundreds of K/s. Forexample, in some embodiments of the methods, cooling is carried out at arate of no greater than 500 K/s. This includes embodiments in whichcooling is carried out at a rate of no greater than 100 K/s, furtherincludes embodiments in which cooling is carried out at a rate of nogreater than 10 K/s, still further includes embodiments in which coolingis carried out at a rate of no greater than 2 K/s and still furtherincludes embodiments in which cooling is carried out at a rate of nogreater than 1 K/s. The use of such low cooling rates is advantageousbecause it reduces the thermocapillary forces experienced by theminority phase droplets relative to those experienced by droplets athigher cooling rates. In addition, the low cooling rate enables thefabrication of larger and complex product by regular casting for wideproduction.

Notably, even when the present methods are carried out by adding thenanoparticles to the system in the miscibility gap (i.e., at atemperature at which the alloy exists as two distinct phases), thenanoparticles provide a more uniform distribution of the minority phaseparticles in the majority phase matrix relative to that provided bylarger particles, such as particles having an average diameter of 1 μmor greater. This may be attributed to the formation by the nanoparticlesof a more uniform and continuous coating on the minority phase dropletsand, therefore, a more effective barrier against droplet coalescence.

The size of the minority particles in the immiscible alloy will dependon a number of factors, including the size and concentration of thenanoparticles and the concentration of the minority phase material inthe alloy. In order to achieve a solid alloy with a highly uniformspatial distribution of minority phase particles, it is desirable tomaintain a minority phase droplet size that is small enough to avoidsignificant droplet sedimentation on the time scale of the alloysolidification process. In some embodiments of the immiscible alloys,the average diameter of the distribution of minority phase particles isno greater than about 11 μm and no more than 1% of the minority phaseparticles have a diameter larger than 20 μm. This includes embodimentsin which the average diameter of the distribution of minority phaseparticles is no greater than about 10 μm and no more than 1% of theminority phase particles have a diameter larger than 15 μm and furtherincludes embodiments in which the average diameter of the distributionof minority phase particles is no greater than about 4 μm and no morethan 1% of the minority phase particles have a diameter larger than 6μm. For example, the average diameter of the distribution of minorityphase particles may lie in the range from about 2 μm to about 10 μm.Because the particles may not be perfectly spherical or symmetric, thediameter of a particle may be measured as its longest cross-sectionaldimension.

The solid immiscible alloys can have a highly uniform minority phaseparticle distribution. That is, the spatial distribution/density ofminority phase particles in the alloy may be highly homogeneous whenmeasured as a function of both the depth and width through the alloysample, where depth and width represent two perpendicular dimensionsextending through the sample. This is illustrated in FIGS. 2B-2F, asdescribed in detail in the Example section. The uniformity of theminority phase particle distribution can be measured by the averagediameter of the minority phase particles measured along the depth andwidth of the alloy and/or by the area fraction of the minority phaseparticles measured along the depth and width of the alloy. For example,in some embodiments of the immiscible alloys, the area fraction of theminority phase particles varies by no more than about ±5%, and in someembodiments no more than about ±3%, when measured along both the depthand the width dimensions of the sample with measurements taken atcross-sectional intervals of, for example, about 2 mm. This includesembodiments in which the area fraction of the minority phase particlesvaries by no more than about ±2%, as well as embodiments in which itvaries by no more than about ±1%, when measured along both the depth andthe width of the sample with measurements taken at cross-sectionalintervals of, for example, about 2 mm. Similarly, in some embodiments ofthe immiscible alloys, the average diameter of the minority phaseparticles varies by no more than about ±3 μm when measured along boththe depth and the width of the sample with measurements taken atcross-sectional intervals of, for example, about 2 mm. This includesembodiments in which the average diameter of the minority phaseparticles varies by no more than about ±2 μm, as well as embodiments inwhich it varies by no more than about ±1 μm, or by no more than about±0.5 μm, when measured along both the depth and the width of the samplewith measurements taken at cross-sectional intervals of, for example,about 2 mm. These uniform minority phase particle distributions can bemaintained even over the dimensions of large alloy samples, includingthose with depth and width dimensions of at least 10 mm, at least 20 mm,at least 50 mm or at least 100 mm.

EXAMPLE

In this example, Al—Bi alloys were used as a model system for immisciblealloys to demonstrate the effectiveness of the present methods. Thedensity difference between Al and Bi is very large (3.6:1), making itone of the most difficult immiscible alloys to process. This exampleshows that, by using thermally stable TiC_(0.7)N_(0.3) nanoparticles(˜80 nm), a uniform distribution of micron-sized minority Bi phaseparticles in an Al matrix can be formed using a cooling rate as low asabout 1 K/s and a Bi concentration as high as 20 wt. % in the immiscibleAl—Bi alloys.

Materials and Methods:

Melt preparation and nanoparticle dispersion: an Al—Bi melt was preparedby melting commercial pure aluminum (99.5%, AA1350) and bismuth (99.9%,Alfa Aesar) in an alumina crucible using an electrical resistancefurnace. TiC_(0.7)N_(0.3) nanoparticles with a diameter of less than 150nm and an average diameter of about 80 nm were fed and dispersed intothe molten metal by an ultrasonic cavitation based method as describedin Y. Yang, J. Lan, X. C. Li, Study on bulk aluminum matrixnano-composite fabricated by ultrasonic dispersion of nano-sized SiCparticles in molten aluminum alloy. Mater. Sci. Eng. A, 380, 378-383(2004). A resistance heating furnace was used to heat the Al—Bi melt inan alumina crucible with a diameter of 36 mm. An inert gas protectionsystem was used to protect the melt and nanoparticles from oxidation byinjecting argon through two nozzles. A nanoparticle feeding system wasused to feed nanoparticles into the melt. In this study, adouble-capsulate feeding method was used. Nanoparticles were wrappedwith 0.0127 mm thin Al foil (alloy 1000), and then the Al foil wasrolled into a rod shape with a diameter of about 6 mm (making the firstcapsule). The Al foil rod, containing the nanoparticles, was wrappedagain with another thin Al foil (alloy 1100) with dimensions of 355.6mm×152.4 mm×0.0254 mm (making the second capsule). The second Al foilmade the nanoparticles discharge into the melt gradually, resulting froma gradual melting of the thicker wall of the capsule. The ultrasonicprocessing system included an ultrasonic probe, booster, and transducer.The ultrasonic probe, made of niobium alloy C103 with a diameter of 12.7mm and a length of 175 mm, was attached to the booster (Sonicator 3000,Misonix Inc.), which was mounted in the transducer working under afrequency of 20 kHz and a maximum 600 W power output. When the Al—Bialloy was melted in the alumina crucible at 973 K, the tip of theniobium ultrasonic probe was inserted to a depth of about 6 mm into themelt. Ultrasonic vibration with a frequency of 20 kHz and a peak-to-peakamplitude of 50 μm was generated from the transducer. Then the preheatednanoparticles (at 423 K for one hour) were added into the melt with thedouble-capsulate feeding method during ultrasonic processing. Eachdouble-capsulate could carry about 1 vol. % of nanoparticles into themelt. It took 15 minutes to disperse the nanoparticles released from onecapsulate by the ultrasonic processing. After feeding and dispersing thenanoparticles at 973 K, the ultrasonic probe was lifted out of the melt.Then the melt was heated to 1183 K (above the miscibility gap for thiscomposition). To make sure the nanoparticles were well dispersed beforecooling, the melt was treated again by ultrasonic processing for 15minutes at 1183 K.

Solidification and cooling rate measurement: After ultrasonicprocessing, the ultrasonic probe was lifted out of the melt. Then thecrucible was taken out of the furnace. The Al—Bi melt with nanoparticleswas cooled down inside the crucible in air. For comparison, the pureAl—Bi alloy was also prepared under the same conditions withoutnanoparticles. The cooling rate was measured by inserting a thermocoupleinside the melt during cooling. The cooling rate before solidificationwas about 1 K/s.

Structure characterization: After solidification, the distribution ofthe minority Bi phase was studied by FESEM (field-emission scanningelectron microscope, Zeiss LEO 1530) using backscattering electronimaging. The area fraction and average size of the Bi phase werecalculated from the backscattering images using the software Image J.The nanoparticle distribution and bonding with the matrix wereinvestigated by FESEM and high resolution transmission electronmicroscopy (HRTEM, FEI Titan Aberration-corrected (S)TEM). Due to apreferential etching of Bi by both electropolishing and ion milling, thesamples for HRTEM were cut by a focused ion beam mill (FIB, Zeiss 1500XB CrossBeam Workstation) to reduce the preferential etching.

Sample preparation procedure for nanoparticle distribution study:Interfacial assembly of the TiCN nanoparticles in the solidified sampleswas studied by FESEM. The samples were cleaned by low angle ion millingfor 2 hours with milling parameters of 4 kv voltage, 4 mA current, 15°milling angle, and 360° stage rocking angle. After ion milling, onlyembedded TiCN nanoparticles remained on the sample, which was confirmedby electron energy-dispersive X-ray spectroscopy (EDS).

Wetting angle determination: To determine the wetting angle of TiCNnanoparticles with Bi in the environment of Al a TEM sample was cut fromthe boundary between Bi and Al by FIB and the triple junction formed bythe TiCN nanoparticle, Bi and Al was observed by HRTEM. The wettingangle of TiCN nanoparticles with Bi in the environment of Al, beforecompleted solidification, was estimated from the geometry of the triplejunction, as shown in FIG. 5.

The dispersion and size of the Bi phase particles in the solidifiedsamples was analyzed. FIG. 2A shows the backscattering scanning electronmicroscope (SEM) images taken from the bottom part of the solidifiedmonolithic Al-20 wt. % Bi (hereafter Al-20Bi) alloy. As expected, the Biphase settled down to the bottom, forming a thick layer of Bi with about12 vol. % of precipitated Al phase. The thickness of the Bi layer on thebottom is about 2.5 mm, accounting for about 90% of the total Bi in thesample. This illustrates that the Bi in the monolithic Al-20Bi alloysegregates severely during solidification. However, when ultrasonicprocessing was used to disperse 2 vol. % TiC_(0.7)N_(0.3) nanoparticlesinto the Al-20Bi alloy, no sedimentation was observed, as shown in theSEM images of FIGS. 2B and 2C. The distribution of the Bi phase in thepure alloy and in samples with 1.0 and 2.0 vol. % of TiC_(0.7)N_(0.3)nanoparticles versus the distance from the bottom of the samples wasalso analyzed, as shown in FIGS. 2D and 2E for average diameters andarea fractions, respectively. With the incorporation of only 1.0 vol. %of TiC_(0.7)N_(0.3) nanoparticles, the distribution of Bi was improveddramatically. With 2.0 vol. % of TiC_(0.7)N_(0.3) nanoparticles, boththe size and area fraction of Bi were very uniform throughout thesample. The average size of Bi droplets was about 7.5 μm. To check ifthe thermocapillary force induced non-uniformity, the distribution of Biphase particles from the center to the outside, along the horizontal(width) direction in the Al-20Bi alloy sample with 2 vol. %TiC_(0.7)N_(0.3) nanoparticles, was analyzed. The results, as shown inFIG. 2F, demonstrated that the Bi phase was distributed uniformly alongthe horizontal direction, indicating that the thermocapillary forceinduced little pushing of Bi droplets to the center. The uniformdistribution of micrometer-sized Bi droplets was observed throughout thesamples of the Al-20Bi alloy with 2 vol. % TiC_(0.7)N_(0.3), which wereproduced under a slow cooling rate of 1K/s.

To study the interfacial assembly of the nanoparticles, the distributionof the TiCN nanoparticles in the solidified samples was investigated byFESEM. As shown in FIGS. 3A and 3B, the nanoparticles form a thin layercoating on a representative Bi droplet.

The stability of the solid nanoparticles assembled at the liquid/liquidinterface was determined by the Brownian motion energy and the energyneeded to remove the particles from the interface to the bulk liquidphases. The energy needed to remove a particle from the interface to thebulk liquid phases isΔG=πr ²σ(1±cos θ)²,  (1)where r is the radius of the particle, σ is the interfacial energybetween the two liquids, θ is the contact angle, measured through theliquid 1 on the solid particle in the environment of liquid 2, and sign‘+’ refers to particle removal into the bulk liquid 2 phase, while sign‘−’ refers to the removal of the particle into the bulk liquid 1 phase.(See, G. Kaptay, On the equation of the maximum capillary pressureinduced by solid particles to stabilize emulsions and foams and on theemulsion stability diagrams. Colloids Surf., A, 282, 387-401 (2006).)The Brownian motion energy (E_(b)) isE _(b) =kT/2,  (2)where k is Boltzmann constant (1.3806503×10⁻²³ m² kg s⁻² K⁻¹) and T isthe absolute temperature of the liquids. In the system of Al—Bi alloywith TiCN particles, the average nanoparticle radius, r, is about 40 nm,interfacial energy σ between Al and Bi liquid is about 16 mN/m at 1173K, and T is about 1173 K. The wetting angle was experimentally estimatedto be about 128°. Thus, the energy (ΔG) needed to remove thenanoparticle from the interface to the bulk liquid phases was estimatedto be about 1×10⁻¹⁷ J, which is more than 3 orders of magnitude higherthan the Brownian motion energy E_(b), 8×10⁻²¹ J, of the particles at1173 K. This indicates that a deep interfacial energy well effectivelytraps the TiCN nanoparticles at the interfacial region between the Biand Al phases, as shown schematically in FIG. 3C.

The diffusional growth of the Bi droplets with and without thenanoparticle coating was also investigated. A detailed description ofthe theoretical model used to investigate diffusional growth is providedin the appendix section, below. For convenience, a summarizeddescription is presented in the following discussion.

Assuming the Bi droplets nucleate and grow only by diffusional transportof Bi, the diameter of Bi can be determined byd _(Bi)=2√{square root over (2DSt)}.  (3)where D is the diffusion coefficient, t is cooling time in theimmiscible region, and S is supersaturation, a temperature dependentparameter. (See, J. Alkemper, L. Ratke, Concurrent Nucleation, Growthand Sedimentation During Solidification of Al—Bi Alloys. Z. Metallkd.,85, 365-371 (1994).) The average S can be calculated by

$\begin{matrix}{{S = {\frac{1}{2}\frac{C_{0} - C_{1}}{C_{2} - C_{0}}}},} & (4)\end{matrix}$where C₀ is the Bi concentration in the matrix, C₁ and C₂ are theconcentrations of Al rich liquid and Bi rich liquid at monotectictemperature, respectively.

By using Equation (3), the estimated average Bi droplet size throughdiffusional growth only would be about 821 μm for pure Al-20Bi, which istwo orders of magnitude larger than the experimental value of 7.5 μm forAl-20Bi-2 vol. % TiCN. This clearly indicates that it would beimpossible to obtain micrometer-sized Bi droplets at a low cooling rate(about 1 K/s) for pure Al-20Bi alloy without effective control ofdiffusional growth. Thus, the nanoparticles greatly reduced thediffusional growth of the Bi droplets during cooling/solidification.

Assuming a Langmuir adsorption model for nanoparticles to assemble onthe surface of a droplet, a theoretical model was constructed to studythe remarkable diffusional growth control of Bi droplets by TiCNnanoparticles. In the model, f is defined as the diffusional blockingefficiency by the nanoparticle coating, C_(np) as the concentration ofnanoparticles, and t_(np) as the characteristic time (time constant) forfull nanoparticle coverage on the droplets. t_(np) is a function of thesize, density and volume percentage of nanoparticles, nanoparticlestacking factor, nanoparticle adsorption efficiency, and melttemperature. C_(np) and t_(np) can be related by a constant k_(np) as

$\begin{matrix}{{t_{np} = {{k_{np}^{- 1} \cdot C_{np}^{- 1}} = {{\frac{1}{k_{np}C_{np}}.{When}}\mspace{14mu} t_{np}{\operatorname{<<}t}}}},} & (5) \\{R \approx {\sqrt{{2{DSt}_{np}} + {2\left( {1 - f} \right){Dst}}}\sqrt{{\frac{D}{k_{np}} \cdot \frac{S}{C_{np}}} + {2\left( {1 - f} \right){{Dst}.}}}}} & (6)\end{matrix}$

Constant k_(np) and f were estimated experimentally by casting theAl-20Bi-2 vol. % TiCN melt to a copper wedge mold to cool the alloy meltat different cooling rates. The k_(np) and f were determined to beapproximately 11340 s⁻¹ and 0.99987, respectively. The t_(np) forAl-20Bi-2 vol. % TiCN can be estimated as about 4.4.0 ms. The shortt_(np) shows that the nanoparticles can rapidly wrap up the dropletswithin a few milliseconds. The remarkably high diffusion blockingefficiency, f, confirms that the nanoparticle coating is effective tocontrol diffusional growth.

Defining γ=S/C_(np), the dependence of the Bi droplet size on γ wasexperimentally investigated by varying the volume percentages ofnanoparticles (C_(np)) and Bi concentration. Using the k_(np) and fexperimentally determined from Al-20Bi-2 vol. % TiCN samples, the Bidroplet size was determined theoretically for the experimentally studiedγ. The theoretical values matched the experimental ones very well (FIG.3D). To show the influence of diffusion blocking efficiency on dropletsizes, the Bi droplet diameter was also calculated theoretically withf=0 (no nanoparticle and no diffusional growth control) and f=1(assuming the nanoparticle coating totally blocked the diffusion). Thedata show that the nanoparticle-enabled diffusional growth controlobtained the remarkably small Bi droplets (FIG. 3D).

Furthermore, FIG. 3E shows the dependence of the experimental andcalculated Bi droplet diameters on cooling rates for Al-20Bi alloy. Thecooling time was calculated by t=(T_(d)−T_(m))/τ, where T_(d) and T_(m)are the decomposition temperature and monotectic temperature,respectively, and τ is the cooling rate. Without diffusional control(i.e. f=0), the droplet size was large and strongly depended on thecooling rate. However, with a nanoparticle enabled diffusional barrier(f=0.99987), the droplet size was orders of magnitude smaller, anddepended little on cooling rates when the cooling rate was higher than50 K/s (corresponding to an approximate cooling time t=10³ t_(np), shownin FIG. 3E). The average Bi droplet sizes depend on the competitivenature between t_(np) and (1−f)t. With a remarkably high diffusionblocking efficiency of f=0.99987 in this study, at a cooling rate thatwill yield a cooling time of about 40 s (approximately 6 K/s inAl-20Bi), t_(np) starts to dictate the Bi particle size, effectivelyeliminating the need for high cooling rates demanded by conventionalprocessing methods.

The effectiveness of nanoparticles at controlling colliding growth wasalso investigated. The SEM images of FIGS. 4A and 4B show that the TiCNnanoparticles trapped at the surface of Bi droplets can stabilize the Bidroplets against coagulation when two Bi droplets collide with eachother, induced by hydrodynamic motions. Even though some Bi dropletswere pushed against each other during solidification, their nanoparticlecoatings were able to resist the coagulation, as shown in FIG. 4A. TheSEM image and energy dispersive spectroscopy (EDS) analysis of theboundary region between the two small Bi droplets validated that a thinlayer of nanoparticle-stabilized Al separated the two Bi dropletsagainst coagulation, as shown in FIG. 4D. Interestingly, somesubmicrometer-sized Bi droplets, located along the Al grain boundary,were also stabilized by nanoparticle coatings.

Past studies showed that solid particles could stabilize the thin liquidfilm between two droplets. (See, G. Kaptay, On the equation of themaximum capillary pressure induced by solid particles to stabilizeemulsions and foams and on the emulsion stability diagrams. ColloidsSurf., A, 282, 387-401 (2006) and N. D. Denkov, I. B. Ivanov, P. A.Kralchevsky, D. T. Wasan, A Possible Mechanism of Stabilization ofEmulsions by Solid Particles. J. Colloid Interface Sci., 150, 589-593(1992).) The pressure needed to break the thin liquid film depends onsize and packing of particles, wetting angle, and liquid/liquidinterfacial energy. (See, G. Kaptay, On the equation of the maximumcapillary pressure induced by solid particles to stabilize emulsions andfoams and on the emulsion stability diagrams. Colloids Surf., A, 282,387-401 (2006) and N. D. Denkov, I. B. Ivanov, P. A. Kralchevsky, D. T.Wasan, A Possible Mechanism of Stabilization of Emulsions by SolidParticles. J. Colloid Interface Sci., 150, 589-593 (1992).) Here it wasassumed that the Bi droplets were fully covered by a dense single layerof nanoparticles. When two Bi droplets approach close to each other, ananoparticle double layer would form to stabilize the thin liquid Alfilm between them. The pressure generated by the double layers ofclosely packed nanoparticles to stabilize the thin liquid Al film can bedetermined by

$\begin{matrix}{{p = {a\;\frac{2\sigma}{r}\left( {{\cos\;\theta} + z} \right)}},} & (7)\end{matrix}$where r is the radius of the particles, σ is the interfacial energybetween the two liquids, θ is the contact angle, a and z are −4.27 and−0.405, respectively, for a closely packed double layer when θ>90°.(See, G. Kaptay, On the equation of the maximum capillary pressureinduced by solid particles to stabilize emulsions and foams and on theemulsion stability diagrams. Colloids Surf., A, 282, 387-401 (2006).)Using parameters for the Al—Bi—TiCN nanoparticle system, the pressurewas estimated to be 3.48 MPa in the liquid Al film between theapproaching Bi droplets, thus effectively resisting colliding growth(coagulations), as shown schematically in FIG. 4C.

Appendix: Theoretical Model for Nanoparticle-Induced Diffusional GrowthControl

Assuming the Bi droplets nucleate and grow only by diffusional transportof Bi, the diameter of Bi can be determined byd _(Bi)=2√{square root over (2DSt)}.  (A1)where D is the diffusion coefficient, t is cooling time in theimmiscible region, and S is supersaturation given by

$\begin{matrix}{S = \frac{C_{0} - C_{\alpha}}{C_{\beta} - C_{0}}} & \left( {A\; 2} \right)\end{matrix}$where C_(α) is the Bi concentration at the interface between the growingBi droplet and Al matrix, C_(β) is the Bi concentration in the Bidroplets, and C₀ is the Bi concentration in the matrix. S is atemperature dependent parameter: S is zero when the cooling just entersthe immiscibility gap; when at the monotectic temperature, S can becalculated by

$\begin{matrix}{{S = \frac{C_{0} - C_{1}}{C_{2} - C_{0}}},} & ({A3})\end{matrix}$where C₁ and C₂ are the concentrations of Al⁻ rich liquid and Bi⁻ richliquid at the monotectic temperature, respectively. The average S canthus be calculated by

$\begin{matrix}{S = {\frac{1}{2}{\frac{C_{0} - C_{1}}{C_{2} - C_{0}}.}}} & ({A4})\end{matrix}$

The following assumptions were made to simplify the study:

-   -   Nanoparticles are uniformly dispersed above the immiscibility        gap;    -   Bi droplets nucleate and grow only by diffusional transport;    -   No coagulation between the Bi droplets occurs; and    -   A densely packed single layer of nanoparticles covers the Bi        droplets.

Since the supersaturation

${S = {\frac{C_{0} - C_{\alpha}}{C_{\beta} - C_{0}} ⪡ 1}},$the growth rate of Bi is

$\begin{matrix}{\frac{dR}{dt} = {D \cdot S \cdot \frac{1}{R}}} & \left( {A\; 5} \right)\end{matrix}$where R is the radius of Bi droplets, t is time, and D is the diffusioncoefficient of Bi in the liquid Al-20Bi alloy. If a constant S isassumed during phase transformation, then

$\begin{matrix}{\frac{RdR}{dt} = {{DS}.}} & \left( {A\; 6} \right)\end{matrix}$

If the nanoparticle surface coverage on the Bi droplets is defined as θand the efficiency of the nanoparticle coating to block the diffusiongrowth of droplets is defined as f (called diffusional blockingefficiency hereafter), then

$\begin{matrix}{\frac{RdR}{dt} = {{{DS}\left( {1 - {f\;\theta}} \right)}.}} & \left( {A\; 7} \right)\end{matrix}$

To obtain θ, assume R>>d, a very densely-packed nanoparticle layer (dueto various sizes of nanoparticles) with an effective packing factor ϕ, dis the layer thickness for the nanoparticle coating. Assuming a Langmuiradsorption model, the additional nanoparticle coverage, dθ, can beattributed to those nanoparticles that could arrive at the interfacewithin a short period, dt, under Brownian motion. To simplify the case,dθ represents the accumulative surface coverage by these additionalnanoparticles during dt. The schematic of the model is shown in FIG. 6.The model can then be described as4πR ² d·φ·dθ=4πR ²(1−θ)·δ·v _(np) C _(np) dt  (A8)where V_(np) is the kinetic velocity of nanoparticles at temperature T,C_(np) is the concentration of nanoparticles, δ is nanoparticleadsorption efficiency.

$\begin{matrix}{\frac{d\;\theta}{dt} = {\frac{\delta\; v_{np}C_{np}}{\varphi\; d} \cdot \left( {1 - \theta} \right)}} & ({A9})\end{matrix}$

A time constant t_(np) can be defined as

$\begin{matrix}{t_{np} = {\frac{\varphi\; d}{\delta\; v_{np}C_{np}}.}} & ({A10})\end{matrix}$

The Brownian velocity of a single nanoparticle can be calculated by

$\begin{matrix}{{\frac{1}{2}{mv}_{np}^{2}} = {\frac{1}{2}k_{B}{T.{Then}}}} & \left( {A\; 11} \right) \\{v_{np} = {\sqrt{\frac{k_{B}T}{m}} = {\sqrt{\frac{k_{B}T}{\frac{4}{3}\pi\frac{d^{3}}{8}\rho}} = \sqrt{\frac{6k_{B}T}{\pi\; d^{3}\rho}}}}} & \left( {A\; 12} \right)\end{matrix}$where m is the mass of nanoparticles, k_(B) is the Boltzmann constant,and ρ is the density of the nanoparticle. Thus,

$\begin{matrix}{t_{np} = {\frac{\varphi\; d}{\delta\; v_{np}C_{np}} = {\frac{\sqrt{\pi}\varphi\; d^{\frac{5}{2}}\rho^{\frac{1}{2}}}{\delta\sqrt{6k_{B}T}}{C_{np}^{- 1}.}}}} & ({A13})\end{matrix}$

From Equation (A13), t_(np) is a function of the size, density andvolume percentage of nanoparticles, nanoparticle stacking factor,nanoparticle adsorption efficiency, and melt temperature.

The Equation (A9) becomes

$\begin{matrix}{\frac{d\;\theta}{dt} = {\frac{1}{t_{np}}{\left( {1 - \theta} \right).}}} & ({A14})\end{matrix}$

The solution of θ can be written as

$\begin{matrix}{\theta = {1 - {e^{\frac{1}{t_{np}}t}.}}} & ({A15})\end{matrix}$

By combining Equations (A7) and (A15),

$\begin{matrix}{\frac{RdR}{dt} = {{DS} \cdot {\left( {1 - f\; + {fe}^{\frac{1}{t_{np}}t}} \right).}}} & ({A16})\end{matrix}$

Assuming constant D and S during the diffusional growth, it becomes

$\begin{matrix}{R^{2} = {{2{{DS} \cdot t_{np} \cdot \left( {1 - e^{\frac{1}{t_{np}}t}} \right)}} + {2{{DS}\left( {1 - f} \right)}{t.}}}} & ({A17})\end{matrix}$

A constant S can be assumed to be given by equation (A4).

Defining:

$\begin{matrix}{{k_{np}^{- 1} = \frac{\sqrt{\pi}\varphi\; d^{\frac{5}{2}}\rho^{\frac{1}{2}}}{\delta\sqrt{6k_{B}T}}},} & ({A18})\end{matrix}$from Equation (A13), K_(np) would be a constant.

Then

$\begin{matrix}{{t_{np} = {{k_{np}^{- 1} \cdot C_{np}^{- 1}} = {{{\frac{1}{k_{np}C_{np}}.{When}}\mspace{14mu} t_{np}} ⪢ t}}},} & ({A19}) \\{{R \approx \sqrt{{2{DSt}_{np}} + {2\left( {1 - f} \right){DSt}}}} = {\sqrt{{\frac{D}{k_{np}} \cdot \frac{S}{C_{np}}} + {2\left( {1 - f} \right){DSt}}}.}} & \left( {A\; 20} \right)\end{matrix}$

Defining:

$\begin{matrix}{{\gamma = \frac{C_{np}}{S}},{then}} & ({A21}) \\{d_{Bi} = {2{\sqrt{{\frac{D}{k_{np}}\gamma^{- 1}} + {2\left( {1 - f} \right){DSt}}}.}}} & \left( {A\; 22} \right)\end{matrix}$

Equation (A22) only considers diffusional growth before the monotectictemperature. At and below the monotectic temperature, small Bi dropletsmay still form. But those Bi droplets formed at and after T_(m) shouldnot be counted so that the experimental data can be compared moreaccurately with the one predicted from Equation (A22). To properlyexclude the small Bi droplets formed at and below the monotectictemperature, the Bi droplet size was checked in the monotectic alloywith 2 vol. % TiCN sample. Droplets smaller than the monotectic oneswere excluded from the SEM images and only the Bi concentration at theimmiscible region was considered in the comparison with the calculationin FIG. 3D.

Experimental determination of k_(np), t_(np) and f: To experimentallyestimate k_(np) and f, the Al-20Bi-2 vol. % TiCN melt was cast to acopper wedge mold to cool the alloy melt at different cooling rates. Thecooling rate was calculated using an empirical equation from thethickness of the sample. (See, N. H. Pryds, X. Huang, The effect ofcooling rate on the microstructures formed during solidification offerritic steel. Metall. Mater. Trans. A, 31A, 3155-3166 (2000).) Thevalues for k_(np) and f were calculated from the droplet sizes obtainedat different cooling rates using equation (A20). The parameters used tocalculate k_(np) and f are listed in table 1, which is presented in FIG.7.

The calculated k_(np) and f were about 11340 s⁻¹ and 0.99987respectively. Using equation (A19), the calculated t_(np) for Al-20Bi-2vol. % TiCN was about 4.4.0 ms.

The parameters used for theoretical calculation of Bi size are shown inTable 2, which is presented in FIG. 8.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”. Still further, the use of “and” or “or” is intended to include“and/or” unless specifically indicated otherwise.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method of making a solid immiscible alloymaterial comprising: an immiscible alloy having a majority phasecomprising a continuous matrix of a first inorganic material and aminority phase comprising a plurality of particles of a second inorganicmaterial dispersed in the majority phase; and nanoparticles comprising athermally stable material coating the surfaces of the dispersed minorityphase particles; the method comprising: (a) forming a single-phaseliquid solution comprising the first inorganic material and the secondinorganic material at a temperature above the liquid miscibility gap forthe immiscible alloy; (b) introducing solid nanoparticles comprising athermally stable material into the single-phase liquid solution while itis at a temperature above the liquid miscibility gap; (c) cooling theliquid solution through the liquid miscibility gap, whereby thenanoparticles form a coating on liquid droplets of the minority phasethat forms in a liquid matrix of the majority phase; and (d) cooling theresulting material to a temperature below the solidus temperature,whereby the solid immiscible alloy material forms.
 2. The method ofclaim 1, wherein the step of cooling the liquid solution through theliquid miscibility gap is carried out at a cooling rate of no greaterthan about 500 K/s.
 3. The method of claim 1, wherein the minority phaseparticles have an average diameter of no greater than about 20 μm andthe average smallest diameter for the distribution of the thermallystable nanoparticles is no greater than about 250 nm.
 4. The method ofclaim 3, wherein the nanoparticles account for about 0.1 to about 5 vol.% of the solid immiscible alloy.
 5. The method of claim 1, wherein thefirst inorganic material is a metal, the second inorganic material is ametal or metalloid and the thermally stable material is a ceramic. 6.The method of claim 5, wherein the immiscible alloy is an aluminum-basedalloy.